The invention applies to matrix screens of the type formed by assembling several elementary screens, each elementary screen comprising a matrix of active elements or pixels. More precisely, the invention is a method of facilitating and simplifying the alignment of the pixels and the splicing of the elementary screens.
Matrix arrangements are frequently used in screens to detect various types of radiation and in flat display screens.
The manufacture of certain relatively large matrix screens poses major problems to the extent that they are often produced by assembling several elementary circuits or screens. Such problems are encountered particularly with C.C.D. (Charge-Coupled Device) image detector screens, each detector screen consisting of an assembly of several elementary detector screens, measuring approx 1.5 mm by 1.5 mm.
The normal method of producing a matrix screen as described above, i.e. using several smaller screens referred to as elementary screens, involves, as a first step, cutting each elementary screen as close as possible to the active zone containing the active elements or pixels. The non active zone remaining on each elementary circuit must be as small as possible to avoid increasing the space between two rows or columns of pixels on adjacent spliced elementary screens compared to the space between rows or columns formed in a given elementary screen. Cutting the elementary screen close to the active zone requires extremely accurate alignment and is, therefore, a long, delicate operation.
The next step is to index the elementary screens relative to each other so that the lines and rows of pixels on each elementary screen are aligned with those on the neighboring screen (to within the required degree of accuracy) to respect the geometry of the image detected or displayed. This is a further delicate alignment operation which requires complex tools.
Three cutting processes are normally used: cutting with a diamond-tipped blade, cleaving the substrate and cutting by chemical etching.
Cutting with a rotary diamond-tipped sawblade generally has several disadvantages:
the heat and mechanical stresses induced interfere with the properties of materials close to the cutting line, deteriorating the electrical properties of the active circuit and imposing an inactive zone whose width is at least that of the zone affected by the cutting operation; PA1 the substrate tends to chip along the cutting line and this must be allowed for in determining the width of the inactive zone between the cutting edge and the edge of the active zone; PA1 a risk of contaminating the sensitive surface (cutting dust, cooling fluids, etc.); PA1 absolute control of the cutting process and reliable, repeatable equipment is required to achieve the necessary accuracy (the specifications are at the limit of what can be achieved in the prior art).
Cleaving the circuit or substrate is only possible if the circuit is produced on a monocrystalline substrate whose orientation was selected to allow cleaving parallel to the matrix directions. This technology limits both the substrate used, i.e. the type of screen, and the matrix production process (the patterns must be aligned with the crystal orientations).
Finally, chemical etching is a relatively slow method, not really compatible with industrial requirements.
The patterns, i.e. the pixels on the various screens, are generally aligned by optical sighting. This method requires special equipment which allows circuits to be moved relative to each other. The precision of the movement along all three axes (X, Y and Z) must be compatible with the size of the patterns being aligned (for example precision better than 10 mm is required for 100 mm patterns).
The indexed positions must then be retained until the elementary screens are attached to a support (for example by bonding). The indexing equipment must, therefore, fulfil a further function.
This method can be applied, with no major difficulty, to small (&lt;20 mm) elementary screens but becomes particularly onerous for screens measuring some tens of mm along one side.
For example, the method is virtually unusable to produce large (for example, 40 cm.times.40 cm) X-ray detection panels for use in radiodiagnostics.
As an example, X-ray detector panels are generally based on an insulating substrate, particularly glass, on which a matrix of amorphous silicon photosensitive elements, for example photodiodes, is produced using thin-film techniques.
These photosensitive elements form a matrix which is then exposed to light from a scintillating screen that converts X-rays into light. The pitch between the rows and columns is, for example, 100 mm. X-ray detector screens, of this type, measuring approx 200.times.200 mm, are manufactured routinely.
However, serious problems are encountered in producing larger screens, particularly due to the fragility of the substrate if glass is used. It is, therefore, very penalizing not to be able to use the elementary screen assembly technique described above to produce large X-ray detector panels from several panels measuring, for example, 200 mm.times.200 mm and each forming an elementary screen spliced along its edges.